Key Takeaways & Executive Findings
- •• • Mg2+ doping suppresses Jahn-Teller distortion by shortening Mn–O bond length, as evidenced by charge density analysis, leading to enhanced structural stability and capacity retention of 90.4% after 200 cycles at 0.1 C. • • The NFMPP-Mg@CNTs cathode achieves a discharge capacity of 126 mAh g−1 at 0.1 C, close to the theoretical capacity of 129 mAh g−1, and maintains 80% capacity after 3000 cycles at 0.5 C, demonstrating excellent long-term cycling stability. • • Mg2+ doping widens Na+ diffusion channels, reducing the migration barrier from 0.566 eV to 0.398 eV, as calculated by first-principles, which enhances Na+ diffusion kinetics and ionic conductivity. • • The material delivers an energy density of 401 Wh kg−1, among the highest reported for mixed phosphate cathodes, positioning it as a strong candidate for large-scale energy storage applications.
Abstract
Manganese-iron-based mixed polyanionic cathodes are promising for sodium-ion batteries (SIBs) due to high energy density and operating voltage, but suffer from Jahn-Teller distortion of Mn3+ that degrades cycling stability. Here, a structural modulation strategy via Mg2+ doping is reported. Electrochemically inert Mg2+ forms stronger chemical bonds, adjusts lattice parameters, and suppresses Jahn-Teller distortion, enhancing structural stability. Mg2+ also widens sodium-ion diffusion channels, improving diffusion kinetics. Additionally, an in-situ three-dimensional carbon nanotube (CNT) conductive network boosts electronic conductivity. The resulting NFMPP-Mg@CNTs cathode delivers a discharge capacity of 126 mAh g−1 at 0.1 C (near theoretical 129 mAh g−1), retains 80% capacity after 3000 cycles at 0.5 C, and achieves an energy density of 401 Wh kg−1, among the highest reported for mixed phosphate systems. Ex-situ XPS and first-principles calculations confirm that Mg2+ resists geometric distortion by enhancing lattice stability and widening Na+ diffusion pathways (migration barrier reduced from 0.566 to 0.398 eV). This work provides a viable route for high-energy, long-life SIB cathodes suitable for large-scale energy storage.
1. Introduction
Sodium-ion batteries (SIBs) are increasingly recognized as viable alternatives to lithium-ion systems for large-scale energy storage, owing to the abundance and low cost of sodium. Among cathode candidates, iron-based mixed phosphate Na4Fe3(PO4)2(P2O7) (NFPP) offers low cost and environmental friendliness but suffers from low energy density, limiting its commercial deployment. To address this, substituting manganese (Mn) for iron raises the redox potential, thereby increasing energy density. However, high Mn content triggers the Jahn-Teller effect, causing severe capacity fade and poor cycling stability, while also compromising electronic conductivity. These bottlenecks have hindered the practical application of Mn-rich phosphate cathodes.
This work introduces a dual modification strategy: doping with electrochemically inert Mg2+ to mitigate Jahn-Teller distortion and enhance structural stability, and constructing an in-situ carbon nanotube (CNT) network to improve electronic conductivity. The Mg2+ doping strengthens chemical bonds, adjusts lattice parameters, and widens Na+ diffusion channels, as confirmed by first-principles calculations showing a reduced migration barrier from 0.566 to 0.398 eV. The resulting NFMPP-Mg@CNTs cathode achieves near-theoretical capacity, exceptional cycling stability (80% after 3000 cycles), and a high energy density of 401 Wh kg−1, directly addressing the critical trade-off between energy density and cycle life in SIB cathodes.
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Zihao Yang, Dongzhu Liu, Yongtao Ma, Xuexia Song, Jingjing Wang, Yanyan Cao, Zhaowen Chen, Xuan Yang, Jiangtao Wang, Xiangyang Xie, Wei Huang, Yukun Xi, Ningjing Hou, Xiaoxue Wang, Wenbin Li, Xifei Li (2026). Pinning effect mitigating Jahn-Teller distortion of manganese-rich phosphate cathodes in sodium-ion batteries. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-026-4132-y
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Frequently Asked Questions
What is the specific mechanism by which Mg2+ doping suppresses the Jahn-Teller effect in Mn-rich phosphate cathodes?
Mg2+ doping shortens the Mn–O bond length, as revealed by charge density distribution analysis. This increases the overlap between Mn d-electrons and O electron clouds, reducing electron cloud asymmetry and thereby suppressing the Jahn-Teller distortion. This structural stabilization is confirmed by ex-situ XPS and first-principles calculations.
How does Mg2+ doping affect sodium-ion diffusion kinetics, and what is the quantitative evidence?
Mg2+ doping widens the Na+ diffusion channels, as shown by first-principles calculations. The migration barrier energy for Na+ along the 1D diffusion path is reduced from 0.566 eV to 0.398 eV, indicating enhanced diffusion kinetics. This is corroborated by GITT and EIS measurements showing lower activation energy and higher ionic conductivity.
What are the long-term cycling stability and capacity retention of the NFMPP-Mg@CNTs cathode at practical current rates?
The cathode retains 80% of its initial capacity after 3000 cycles at 0.5 C, demonstrating excellent long-term stability. At 0.1 C, it delivers a reversible capacity of 126 mAh g−1 with 90.4% retention after 200 cycles, surpassing previously reported mixed phosphate cathodes.
How does the energy density of this material compare to other mixed phosphate cathodes, and what is the industrial significance?
The NFMPP-Mg@CNTs cathode achieves an energy density of 401 Wh kg−1, which is among the highest reported for mixed phosphate systems. This high energy density, combined with long cycle life, makes it a strong candidate for large-scale energy storage applications where cost and longevity are critical.
What is the role of the carbon nanotube network in the electrode performance, and how is it integrated?
The in-situ three-dimensional CNT network significantly improves electronic conductivity, reducing charge transfer resistance and facilitating fast electron transfer. This integration enhances the rate capability and overall electrochemical performance, as evidenced by improved reaction kinetics and capacity retention.
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